MOD 07 - Gypsum (Type III)

Dental Stone

• Type III, Alpha form (Caso4.1/2h2o)

• high strength stone

• calcined by 30% calcium chloride solution

• construction of casts, fabrication of full dentures to fit soft tissues

• stronger and more expensive than plaster

• more uniform, dense, prismatic shape in the form of

  • mold and prism

• if mixed with water, dental stone sets harder mass

Dental stone is produced in two methods which are:

1. Heated 125 degrees under steam pressure in autoclave (more regular and less poroud)

2. Boiled in colution of salt Cacl2 (less porosity, produced by autoclaving)

Dental stone is used for

1. making cast

2. diagnostic purpose

3. cast for rpd & cd

4. processing dentures

Wet calcination of dental stone

aLpha calcium sulfate hemihydrate is formed smaller, regularly shaped, crystalline particles in form of rods and prism

• type iii dental stone or hydrocal or clas 1 stone

wp ratio of dental stone

0.28-0.30

water ratio for dental stone

29-35

powder ratio for dental stone

100

advantages and disadvantages of dental stone

Advantages is greater strength and surface hardness while the disadvantage is it is more expensive than plaster

Fill out the property requirements for each gypsum products

Fill out the setting time for each gypsum products

Fill out the compressive strength for each gypsum products

Factos controlling setting time

1. Factors under manufacturers

2. Factors under control of the operator

Factors under manufacturers

1. Concentration of nucleating agents in hemihydrate powder. more nucleating = more crystallization

2. Addition of chemical accelerators or retarders to dental stone

Factors under control of operators

1. Temperature

2. Increasing W:P

3. Increasing mixing time

Setting expansion of Type I, II, III

• Type I - 0-0.15

• Type II - 0-0.30

• Type III - 0-0.20

To produce an accurate model or die, you should maintain

setting expansion at low value

Compressive strength

Depends on porosity, time which material allowed to dry out

Flexural strength

• Stone is less fragile but must be treated with care

• likely to fracture if droped

Flexural strength of Types I, II, III

• Type I - 1

• Type II - 1

• Type III - 15

Detail reproduction

• Reproduce surface detail of hard and soft tissue either directly or from impression

• Strength, hardness, accuracy requires dental stone

• stone are most likely to be damaged during laying down & caring of wax pattern & give optimal dimensional accuracy

Detail reproduction of types I, II, III, IV, V

• Type I - 75

• Type II - 75

• Type III - 50

• Type IV - 50

• Type V - 50

What number of detail reproduction is capable of greater fine details

50. Type III, IV, V